Integrated circuits often contain self-biasing circuits that have two or more stable states of operation or convergence points, wherein one state is the desired operational state. Such self-bias circuits typically utilize a feedback circuit in their operation and therefore require a start-up circuit to initiate the desired state of operation at the proper convergence point upon circuit power-up. These self-bias circuits include, but are not limited to band-gap voltage reference circuits, current references, A/D converters, D/A converters, and feedback circuits.
Most self-bias circuits, such as band-gap voltage reference circuits, have two stable states of operation. Typically one state is the desired operation state and the other is a zero-current state. To prevent the zero-current state from occurring, undesirably, a start-up circuit is typically added to the self-bias circuit, which applies an initiating voltage or injects a starting current or current pulse to the self-bias circuit to initiate operation of the self-bias circuit in the desired state.
ICs and memories are designed to operate over a set range of supply voltages and temperatures. In modern ICs and memories the supply voltages have become increasingly smaller, which in part decreases the power usage in these circuits. As stated above, a problem in many prior art self-bias circuits, such as band-gap voltage references, is that the circuit has at least two stable states of operation. In a band-gap voltage reference circuit these states are where current is flowing in the circuit and the circuit is providing a stable voltage reference and where no current is flowing in the circuit and no voltage reference is being output. Upon power-up of the circuit an unassisted self-bias circuit will assume one of these two states of operation.
However, many of these start-up circuits themselves consume current and dissipate power when not active and become less effective at initializing the self-bias circuit as the supply voltage gets lower. The situation is even more problematic in portable devices as the total power used becomes more of an issue and it becomes important that the start-up circuit must draw as little current as possible during standby or normal operation. Additionally, the steady-state power draw of the start-up circuit after the self-bias circuit has been initialized and start-up circuit is inactive becomes an important factor.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for an improved start-up circuit for self-bias circuits and band-gap references circuits in modern ICs and memory circuits.